1. Field of the Invention
The present invention relates to a preemphasis circuit and a complementary deemphasis circuit for use in a recording/playback apparatus such as a video or audio tape recorder, or in communication equipment.
In recording apparatus as well as in communication equipment, preemphasis is executed on the recording or transmitting side and deemphasis on the playback or receiving side to enhance the signal-to-noise ratio. If, in a preemphasis circuit, which increases the amplitude of a signal component whose frequency is higher than a predetermined frequency, a circuit with a limiter is used, the preemphasis circuit is termed a nonlinear preemphasizer and the signal amplitude to be increased is limited to a predetermined threshold level. The present invention more particularly relates to a system comprising such a nonlinear preemphasis circuit and a complementary nonlinear deemphasis circuit for improving the distortion characteristic normally attributed to nonlinearity.
2. Description of the Prior Art
FIG. 8 is a block diagram of a conventional system consisting of a nonlinear preemphasis circuit and a nonlinear deemphasis circuit. In a video tape recorder as an example, a recording section la includes a high-pass filter 52, a limiter 54, a signal increaser (also referred to as a coefficient multiplier) 56 and an adder 58. The high-pass filter 52 passes frequencies above a predetermined high frequency component of an input signal S.sub.s, and the limiter 54 limits the amplitude of the signal component passed through the high-pass filter 52 to a predetermined threshold level, as graphically shown in FIG. 10. The signal increaser 56 serves to increase the output signal level of the limiter 54 in accordance with the frequency of that signal. Such nonlinear preemphasis circuit 50 prevents, via the limiter 54, further amplification of the output of the high-pass filter 52 beyond the predetermined threshold level. The signal component whose frequency is higher than the predetermined frequency and whose level has been increased by signal increaser 56 is supplied to the adder 58 whereat it is added to the input signal S.sub.s. The summed signal thus obtained is supplied as a recording signal to a recording/playback section 2a, where the signal is recorded on a magnetic tape and subsequently read out therefrom to be reproduced in a playback section 3 a. In this example, the recording/playback section 2a comprises a recording head, a magnetic tape, a playback head and an amplifier.
The playback section 3a includes a nonlinear deemphasis circuit 70 which comprises a subtracter 78, a high-pass filter 72, a limiter 74 and a coefficient multiplier 76 which operate to decrease the amplitude of a signal component in a manner that is complementary to the increase obtained in the recording section 1a. The deemphasis circuit 70 is an inverted version of the nonlinear preemphasis circuit 50.
The circuits in the playback section 3a of FIG. 8 are arranged in a negative feedback loop and exhibit very good deemphasis characteristics. However, due to the negative feedback circuit configuration, high stability is not achievable over a wide frequency range. Furthermore, this circuit configuration is not readily suited to be implemented in an IC configuration.
FIG. 9 shows an improvement of the nonlinear deemphasis circuit of FIG. 8. To solve the problem attributed to the negative feedback circuit configuration in the example of FIG. 8, a nonlinear deemphasis circuit 80 is comprised of a high-pass filter 82, a limiter 84 and a coefficient multiplier 86 which are connected in feedforward configuration to an adder 88. The nonlinear deemphasis circuit formed of high-pass filter 82, limiter 84 and coefficient multiplier 86 is designed to have an operating characteristic that is the inverse to that of the nonlinear preemphasis circuit 50.
The nonlinear deemphasis circuit 80 of FIG. 9 has a flat frequency characteristic with small distortion in the lower frequency range where the amplitude of the preemphasized/deemphasized signal is not limited by the limiters 54 and 84 (this is referred to as the linear area of operation of the preemphasis or deemphasis circuit). However, distortion occurs in the nonlinear area where the signal amplitude is widely limited by the limiters.
FIG. 11 shows the configuration of another nonlinear deemphasis circuit that is intended to solve the above problem of distortion in the nonlinear area of operation. The nonlinear deemphasis circuit 90 in FIG. 11 consists of a parallel connection of two nonlinear deemphasis circuits 90a and 90b which operate on mutually different frequency components. In this example, a signal component in one frequency band passes through a high-pass filter 91 and a low-pass filter 93 in the first deemphasis circuit 90a and then its amplitude is decreased by a coefficient multiplier 94. A signal component in another frequency band passes through a high-pass filter 95 and a low-pass filter 97 in the second deemphasis circuit 90b and then its amplitude is decreased by a coefficient multiplier 98. The two amplitude-decreased signal components in these mutually different frequency bands are combined in an adder 68.
The nonlinear deemphasis circuit 90 shown in FIG. 11 has a feedforward configuration and thus exhibits improved distortion characteristics. However, its circuit configuration is undesirably complicated.